This invention relates to long range ordered alloys of the transition metals V, Fe, Ni, Co, and the like, and more specifically to modification of such alloys by substituting a quantity of Group IV-B metal(s) for a like quantity of V in these alloys to improve mechanical properties at elevated temperatures. This invention was made as a result of a contract with the U.S. Department of Energy.
Long range ordered (LRO) alloys are like intermetallic compounds whose atoms are arranged in order below a critical ordering temperature, T.sub.C. The term "long range order" refers to alloys having ordered structure extending for a distance of more than about 100 atoms in a single domain. Group IV-B metals are Ti, Zr, Hf, and their mixtures as classified in the periodic table of Webster's 3d New International Dictionary, P. B. Gove, Ed., G&C Merriam Co., Springfield, Mass. (1967).
A principal disadvantage associated with high-temperature applications of LRO alloys has been lack of ductility or tensile elongation reducing fabricability due to brittleness. One approach for improving ductility and subsequent applicability to structural components for LRO alloys is disclosed in assignee's U.S. Pat. No. 4,144,059 which issued Mar. 13, 1979, in the name of Chain T. Liu and Henry Inouye, entitled "Ductile Long Range Ordered Alloy With High Critical Ordering Temperature and Wrought Articles Fabricated Therefrom." The Co-based alloys described therein have the nominal composition V(Co,Fe).sub.3 or V(Co,Fe,Ni).sub.3 and comprise by weight 22-23% V, 14-30% Fe, 37-64% Co, and 0-10% Ni. The alloys (LRO-1 to 4) of this patent are relatively expensive due to the high cost limited supply of cobalt and have limited applicability in nuclear environments because of the high neutron-absorption cross section of cobalt.
Another approach to providing ductile LRO alloys is disclosed in assignee's U.S. Pat. No. 4,238,229 which issued Dec. 9, 1980, in the name of Chain T. Liu, et al, entitled "Fe-Based Long Range Ordered Alloy." The Fe-based alloys described therein have the nominal composition V(Fe,Ni).sub.3 or V(Fe,Ni,Co).sub.3 and comprise by weight 22-23% V, 35-50% Fe, 0-22% Co, and 19-40% Ni. These alloys contain less than 22% by weight Co and have superior nuclear properties and lower costs than Co-based alloys of U.S. Pat. No. 4,144,059. The Fe-based alloys also exhibit improved mechanical properties at elevated temperatures. However, these improved properties of Fe-based alloys (LRO-15 to 18) occur at slightly lower temperatures than for Co-based alloys of U.S. Pat. No. 4,144,059.
Inasmuch as the LRO alloys described in the aforementioned patents are essentially the alloys modified by the present invention, these patents are incorporated herein by reference.
While elevated temperature performance of these patented Co and Fe-based LRO alloys is generally several orders of magnitude better than conventional or disordered alloys, their ductility decreases as their critical ordering temperatures (T.sub.C) are approached. Sigma phase embrittlement of alloys of the referenced patents has also been noted at elevated temperatures whenever the Fe content in the alloy composition exceeds about 20% by weight. This sigma phase formation is difficult to remove because the sigma phase retained below T.sub.C cannot be easily annealed out due to the lower T.sub.C of Fe-based LRO alloys.
It will be appreciated that long range ordered alloys of these patented VCo.sub.3 and VFe.sub.3 formulations may be limited in structural and component applications involving high temperature and nuclear environments. In order to reduce these limitations, there is a need for an alloy formulation which will provide improved mechanical properties at elevated temperatures while suppresing brittle sigma phase formation.